tl;dr a deer farm in Michigan was probably cutting it's feed with deer brains so now the deer have a Mad Cow style disease and it's spread to wild deer.
http://www.freep.com/story/sports/outdoors/2017/11/26/hunters-must-change-approach-help-stall-cwd-spreading-humans/894259001/>Of the 11 free-ranging deer identified with CWD in Michigan, five were in Ingham County, four in Clinton County, and two in Montcalm County (three more deer in Montcalm are also suspected to have it). "If those three are recognized as positive, then what took us three years to get to in Ingham County, it will have taken us three weeks to get to in Montcalm," said Stewart.

>Three CWD-infected deer have been identified in agricultural deer facilities, one in 2008 in Kent County and two this year in Mecosta County. These fall into the jurisdiction of the Department of Agriculture, not the DNR, according to Michigan DNR wildlife chief Russ Mason.

>"In 2004 we initiated APRs in 29 counties. The intent was to shift harvest, to apply more pressure to does," Sumners said. "It became the most popular regulation we had in those areas, because within three years, they were seeing larger deer. Seventy-five to 80 percent of hunters liked it. That's a good day when you can get that many hunters to agree on anything."

>Then CWD struck. "We found CWD in 2010 in captive facilities and then in free-ranging deer," Sumners said. "We knew we had to change our approach."

>APRs were rescinded in diseased counties. "Let them go and let them grow" went out the window. Sumner was concerned APR supporters would be dissatisfied. The results surprised him.

>"We compared hunter attitudes between those that have the APRs versus those that are part of CWD management zone where younger deer are targeted. Hunter satisfaction ratings of our deer management were identical."